The present invention relates to a device for producing microstructures with a spindle, which is adapted to be driven for rotation about its longitudinal axis and which is provided with a fixture for clamping a workpiece, having an actuator provided with a fast drive adapted to produce fast movement of a tool in a direction substantially perpendicular to a workpiece surface, which actuator can be positioned along a workpiece surface to be worked with the aid of an additional drive adapted to produce a linear feed motion in a first direction.
The invention further relates to an actuator suited for a device of that kind.
Finally, the invention relates to a method for producing a microstructured surface on a workpiece which can be rotated by a spindle using a tool that can be driven by an actuator and can be positioned in linear direction along a workpiece by means of a drive and can be moved by the actuator toward the workpiece surface in perpendicular direction relative to that linear direction.
A device and a method of the described kind are known from EP 0 439 425 B1.
The known method and the known device are used for producing contact lenses by a turning process. According to that process, a blank for a contact lens to be worked is mounted on a spindle. The workpiece, being rotationally driven by the spindle, can be worked by a turning tool which is positioned with the aid of a piezo drive. The turning machine comprises a column-like swivel head seated for pivotal movement about a pivot axes. A carriage guide is mounted on the swivel head. The carriage guide serves to guide a tool carriage in linear and radial direction relative to the pivot axis of the swivel head. A tool holder for the turning tool, preferably in the form of a turning diamond is supported on the tool carriage. For fast feed of the turning tool about the pivot axis of the swivel head, the tool carriage can be moved along the carriage guide by means of a motor drive. For fine positioning of the turning tool, a piezo drive is provided which may consist of two piezo translation means. While a first piezo translation means allows a movement to be performed in the direction of the carriage guide, a second piezo translation means is adapted to permit movement perpendicular to that first movement. The positioning movements of the piezo translation means may be controlled in response to the angle of rotation of the spindle about the spindle axis.
This arrangement permits highly precise surface working of lenses by a diamond cutter, and permits even rotationally non-symmetrical surfaces to be produced.
Similar devices for producing rotationally non-symmetrical surfaces by turning with the aid of a piezo drive have been known, for example, from U.S. Pat. No. 5,467,675 or GB 2 314 452 A.
Although the known devices and the known methods allow fine-structure surface working using diamond tools, such devices and methods, due to inadequate dynamic properties, are not suited for working very hard metal materials with high precision. Such materials are required, for example, as mold materials for the production of lenses by hot-pressing. The materials in question may, for example, be hard alloys. In the production of lenses for illumination purposes, which are used in spotlights known as poly-ellipsoid spotlights (PES spotlights) it is, for example, necessary to provide such molds with very specific microstructures which are then transferred to the respective lenses during the hot-pressing process. The microstructures serve as micro-optical components in the lenses so produced in order to meet given predefined light intensity distribution characteristics for the spotlight.
The production of such molds, consisting of a hard alloy or of cast iron, for example, has been possible to this day in the envisaged form only with the aid of geometrically uncertain processes. The molds are initially worked in the macroscopic form of the respective lenses by turning, where after the mold surface is polished, if necessary. Thereafter, the areas of the mold from which scattering centers are to be created on the lenses to be produced are produced by a number of operations, for example by a corundum blasting process in which masks are employed to cover those areas on which no microstructures are to be produced in that way. The blasting process is then followed, in part, by a two-dimensional after-treatment. Thus, for producing such a mold, numerous manual operations are required, which means that extremely time-consuming and expensive working is necessary to produce the desired surface structure. In addition, such a sequence of operations is highly susceptible to errors and faults which has an adverse influence on the reproducibility of the given light intensity distribution characteristics.
The before-mentioned known devices with piezo drives are not suited for such shaping operations, lacking the mechanical stability required for that purpose and the dynamic properties necessary for working hard metal piezo workpieces precisely at the required cutting speeds.
While position-controlled working may be possible with known devices, this naturally can be done only with a sufficiently big distance from the first resonant frequency of the respective system, which means that working with the known systems is possible only up to approximately 1,000 Hz maximally. The cutting speeds so achievable are, however, insufficient for working the before-mentioned materials properly, with adequate surface quality, in particular when rotationally non-symmetrical surface structures of the before-mentioned kind are to be produced.